Judges in California, like a few others across the country, have raised concerns about federal use of cellphone tracking devices known as “stingrays,” suggesting that investigators have been using the technology without explaining to judges exactly what they are doing, according to internal Justice Department emails.

Stingrays simulate a cellphone tower and can locate a phone even if it isn’t making a call. A handful of federal judges have now expressed concerns about similar cellphone tracking technologies, particularly because federal officers have been using them after getting lower court orders that don’t meet the same standard as search warrants.

In California, emails written by several U.S. attorneys in 2011 indicate that this has become what they describe as a “problem” in that state as well. One of the emails explains that magistrate judges there have “collective concerns” about whether the court orders, known as pen register orders, are “sufficient to authorize the use” of the technology.

“It has recently come to my attention that many agents are still using [stingray] technology in the field although the pen register application does not make that explicit,” one of the attorneys wrote, adding that the office was working on a “long term fix.”

Pen registers are tools that gather signals from phones such as numbers dialed but don’t receive the content of conversations. To use them, investigators don’t have to show probable cause, the way they would with a search warrant. But pen registers were designed before the widespread use of cellphones, which can transmit more data than landline phones and can be used to pinpoint a person’s location. The template that investigators use when writing pen register requests doesn’t necessarily indicate when the device being used is actually a stingray, the California emails suggest.

The emails came to light as a result of a Freedom of Information Act request filed by the American Civil Liberties Union, which is arguing this week in the case of a man who was arrested after investigators used a stingray to track a mobile broadband card to a California home.

“What these emails show is that investigators are routinely using stingray technology but failing to make that explicit,” said Linda Lye, an attorney with the ACLU. The government “was routinely pulling this stunt, and the magistrates were not pleased,” she said.

The Justice Department did not respond to a request for comment. A spokeswoman for the U.S. District Court in the Northern District of California said it was not possible to immediately determine what concerns and which judges the emails might be referencing.